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Abstract

Samples of the Fort Hays Limestone and lower Smoky Hill Shale Members of the Niobrara Formation were collected from a quarry in Lyons, Colorado. Studies of levels of bioturbation, organic carbon content, carbonate carbon content, and isotopic composition of whole-rock carbonate and inoceramid bivalve shells indicate significant variation in most of these properties through this interval. The oxygen and carbon isotopic compositions of whole-rock carbonate are relatively constant and unusually negative, reflecting the largely recrystallized nature of the fine-grained constituents and a locally elevated thermal history. These factors were of less influence on inoceramid bivalve shells, which appear to have retained isotopic compositions close to primary values. From the uppermost part of the shale and limestone unit through the basal part of the lower limestone unit of the Smoky Hill, there is a marked change in paleoenvironmental conditions indicated by a decrease in extent of bioturbation, increase in organic carbon content, and decrease in oxygen isotopic values of inoceramids. These nearly synchronous changes suggest increasing influence of riverine water and decreasing benthic circulation in the Western Interior seaway during deposition of these rocks.

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